1. Field of the Invention
This invention pertains generally to borocarbide synthesis, and more particularly to methods of synthesizing LiBC and Li1-xBC.
2. Description of Related Art
Layered diboride compounds have been the focus of intense interest since the discovery of superconductivity in MgB2 (see J. Nagamatsu, N. Nakagawa, T. Muranaka, and J. Akimitsu, Nature 410, 63 (2001), incorporated by reference herein). A derived class of materials is the similarly layered borocarbides. Of particular interest are the Li1-xBC compounds where high temperature superconductivity has been predicted for high hole-doping ranges (see, H. Rosner, A. Kitaigorodsky, and W. E. Pickett, Phys. Rev. Lett. 88, 127001 (2002), incorporated herein by reference). FIG. 1 and FIG. 2 generally illustrate the structure of the LiBC material, shown in two and three dimensions, respectively. Synthesis of the LiBC using sealed niobium ampoules has been reported by Wörle et al. (see, M. Wörle, R. Nesper, G. Mair, M. Schwarz, and H. G. von Schnering, Z. Anorg. Allg. Chem. 621, 1153 (1995), incorporated herein by reference), along with initial characterizations. The parent compound LiBC forms a hexagonal lattice with space group P63/mmc. The B—C layers are fully intercalated, with half of the boron ions on alternating sites replaced by carbon ions, both in-plane and along the c-axis (see, M. Wörle, R. Nesper, G. Mair, M. Schwarz, and H. G. von Schnering, Z. Anorg. Allg. Chem. 621, 1153 (1995); M. Wörle and R. Nesper, J. Alloys Compounds 216, 75 (1994); both of which are incorporated herein by reference). Lithium ions occupy the interstices between the parallel B—C layers.
To explore any superconducting behavior in the Li1-xBC compounds, it is essential to achieve clean phases. This is challenging since lithium can be combined with practically every element on the periodic table to form thermodynamically stable compounds (except with the other alkali metals) due to its high electron affinity and strong polarizing power (see, R. Nesper, Prog. Solid. St. Chem. 20, 1 (1990), incorporated herein by reference). The presence of impurity phases, as noted in the early study (see, M. Wörle, R. Nesper, G. Mair, M. Schwarz, and H. G. von Schnering, Z. Anorg. Allg. Chem. 621, 1153 (1995), incorporated herein by reference) can greatly complicate the investigation. Complications increase dramatically in response to magnetism. Another potential complication is the reported weak ferromagnetism in related CaB2C2 (see, J. Akimitsu, K. Takenawa, K. Suzuki, H. Harima, and Y. Kuramoto, Science 293, 1125 (2001), incorporated herein by reference) that could mask or destroy superconductivity. A second key issue is the structural stability of LiBC upon Li de-intercalation, especially at high doping regime. The predicted superconductivity depends sensitively on a minimal lattice distortion (see, H. Rosner, A. Kitaigorodsky, and W. E. Pickett, Phys. Rev. Lett. 88, 127001 (2002), incorporated herein by reference) which is yet to be demonstrated explicitly.
Accordingly, a need exists for a method of synthesizing LiBC and Li1-xBC that is simple and effective while providing clean phases as outlined above. The present invention satisfies those needs, as well as others, and overcomes the deficiencies of previously developed synthesis methods.